OVN update in 16.1 causes network loss in dataplane

Issue

In OSP, you update the Controller nodes before the Compute nodes. However, if you update the Controller nodes first, the non-updated ovn_controller service on the non-updated nodes in the Compute role flush the openflows and drop instance connectivity. As a result, your instances will not have any network connectivity to and from other instances within overcloud project networks or external connectivity with outside hosts. The issue is resolved when you update nodes in the Computes role.

To avoid network outages, you must update OVN on the Compute nodes before you update the Controller role.